The lights are pretty simple. We waited until now so that we are familiar with the movement of the robot and do not obstruct it with our string of lights.
Take your battery powered strand of lights and cut the battery compartment off. (Mark one of the wire so you know which is which)
Strip the two ends and solder them to some breadboard friendly male headers.
Wire the positive wire to Arduino pin 13, and the negative out to ground.
That is all. Now we can control the lights from digital pin 13.
Now that you know your robots moving bits well, you can go ahead and string the lights up. You do not want to obstruct the function of your talking tree, so watch where you hang them. I forgot to take a picture with just the lights, but you can see what it looked like in the video in the next step.